This invention relates to a multitubular heat exchanger provided with a bundle of U-shaped heat transfer tubes and a vent tube for collecting noncondensable gas in a shell, especially to a feedwater heater used in thermal and nuclear power plants.
Feedwater heater used in such a power plant is shown in U.S. patent application Ser. No. 823,655 filed in 1977 "MULTITUBULAR HEAT EXCHANGER" in the name of FURUKAWA et al., assigned to the same asignee of the present application. In the above U.S. Patent Application, a shell forms a steam condensing compartment. A bundle of U-shaped heat transfer tubes are located in the steam condensing compartment. A vent tube is located between an upper portion and a lower portion of the bundle of U-shaped tubes. A steam inlet of the steam condensing compartment is formed at an upper portion of the shell.
Feedwater is introduced in the lower portion of the U-shaped tubes and discharged from the upper portion of the U-shaped tubes. Steam is introduced in the steam condensing compartment through the steam inlet formed at the upper portion of the shell. Steam introduced in the steam condensing compartment, at first, heats feedwater flowing through the upper portion of the U-shaped tubes. Secondarily, steam heats feedwater flowing through the lower portion of the U-shaped tubes.
Accordingly, the temperature of feedwater flowing through the upper portion of the U-shaped tubes is higher than that of feedwater flowing through the lower portion. The ratio of the amount of heat exchanged in the lower portion of the bundle of U-shaped tubes to that in the upper portion of the bundle is about 20:1. A greater amount of steam is condensed into water in the lower portion of the bundle while a less amount of steam is condensed in the upper portion of the bundle. So, a greater amount of steam flows into the lower portion of the bundle, especially toward the middle of the lower portion of the bundle.
The steam includes noncondensable gas, for example ammonia gas which is for avoiding an adherance of scale to an interior of a boiler. Ammonia gas stagnates in the middle of the lower portion of the bundle. In this way, a noncondensable gas stagnating zone is formed there. The U-shaped heat transfer tubes wear away gradually by chemical action, especially at portions of the tubes proximate tube support plates.
Since the vent tube is located between the upper portion and the lower portion of the bundle of the U-shaped tubes, it is difficult to collect noncondensable gas in the noncondensable gas stagnating zone.